Switches in the low-voltage range comprising electromechanical releases are known, for example, as power circuit breakers. The power circuit breakers interrupt the current flowing, for example, when a preset current value, which is detected by an electronic trip unit, is exceeded. The release furthermore comprises a latching magnet (Maglatch), which has a permanent magnet, a winding and a ferromagnetic plunger, wherein the plunger is attracted, counter to the force of a spring, by the magnetic field of the permanent magnet. The tripping takes place by virtue of an opposing magnetic field being generated by the winding arranged on the permanent magnet, which opposing magnetic field compensates for the magnetic field of the permanent magnet in the region of the plunger, such that the plunger is shifted by the spring in the direction of its longitudinal axis in order to actuate the switch. The plunger in this case interacts with an actuator in the form of a switching mechanism, in which a switching shaft is held under mechanical prestress, which switching shaft unlatches the plunger. The switching shaft rotating about its longitudinal axis separates the contact elements and thus opens the switch.
The winding of the latching magnet forms, with an electrical energy store in the form of a capacitor, a circuit which is closed (and opened again) when the tripping condition of the trip unit is met. In general, the capacitor is only partially discharged.
In order to test the latching magnet, i.e. in particular the winding for wire breakage, it is known to pass a test current through the winding which is too low to release the plunger. The test current is monitored, wherein an interruption in the current in particular indicates a wire breakage.